1. Field of the Invention
This invention relates to a method and circuit for detecting a training signal which is transmitted for initial adjustment of the equalizing characteristics of an automatic equalizer provided in a Modem (Modulator-Demodulator).
2. Description of the Prior Art
In a Modem having therein an automatic equalizer, it is a common practice to transmit a training signal, prior to the transmission of message data, to a receiving Modem for initial adjustment of the automatic equalizer with respect to channel characteristics. The CCITT (International Consultative Committee for Telephone and Telegraph) has proposed such a training signal in "Recommendation V.27 bis". This signal consists of three segments which precede the message data in a manner schematically shown in FIG. 1. The first segment is composed of continuous 180.degree. phase reversals for 14 symbol intervals, the second segment of 0.degree.-180.degree. 2-phase equalizer conditioning pattern for 58 symbol intervals, and the third segment of continuous scrambled ONEs for 8 symbol intervals.
Such a training signal however consumes a large amount of time for initial adjustment of the automatic equalizer and hence the efficiency of a data transmission system is seriously limited especially when the adjusting time approaches or exceeds the actual message time.
In order to overcome this problem, another training signal has been proposed in the European Patent Application publication No. A1 0052362. This training signal is composed of at least two periodic data sequences (maximum length sequences) each having a period L and is such that the autocorrelation function of the sequence assumes an impulse. This training signal will be described in more detail hereinlater.